Precision assemblies such as exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly having a reticle stage that retains a reticle, an optical assembly, a wafer stage assembly having a wafer stage that retains a semiconductor wafer, a measurement system, and a control system. Additionally, one or more of the stage assemblies can include a mover assembly that precisely positions the stage(s).
The size of the images and features within the images transferred onto the wafer from the reticle are extremely small. Accordingly, the precise positioning of the wafer and the reticle relative to the optical assembly is critical to the manufacture of high density, semiconductor wafers. Recently, one or more attraction only-type actuators have been used in the mover assemblies. For example, attraction-only type actuators can include one or more E/I core type actuators. The E/I core type actuator can include a somewhat “E” shaped electromagnet and a somewhat “I” shaped target that is positioned near the electromagnet. Among other configurations, attraction-only type actuators can include CI core type actuators which use a “C” shaped electromagnet.
Each “E” shaped electromagnet can have two wings and a center section. An electrical coil is typically wound around the center section of the “E”. Between the electromagnet and the target is a relatively small gap (also referred to herein as the “gap distance”) which can typically range from between zero and 500 microns. Current directed through the coil creates an electromagnetic field that attracts the target toward the electromagnet, effectively decreasing the gap between the electromagnet and the target. The amount of current to be directed to the electromagnet is based at least partially upon the gap distance. The gap distance can be monitored to increase positioning accuracy of the stage, and to reduce the likelihood of a collision between the electromagnet and the target, which could damage components of the precision assembly.
In light of the above, there is a need for a precision assembly that can determine misalignments between the electromagnets and the targets in the mover assemblies. Further, there is a need for a control system that can account for any such misalignments by more precisely controlling the amount of current that is directed to the electromagnets based at least partially on the presence and/or the extent of such misalignments.